In consumable metal inert gas (MIG) arc welding, a consumable wire electrode is continuously fed by a wire feeder to the welding torch from a supply roll. The consumable wire electrode passes through the torch and feeds out through a contact tip a short distance which is called "stick out". In order to start the welding process, the operator presses a switch on the torch which energizes the welding contactor and turns on the power supply. At the same time, the motor for operating the wire feeder is started and the shielding gas begins to flow to the torch. The consumable wire electrode is touched to the workpiece which initiates the arc and the operator begins to weld. However, the operator must continue to hold the switch in the closed position while carrying out the welding operation. This can be rather awkward for the operator if he is going to weld for long periods. Another disadvantage of this conventional system is that the switch adds to the cost of the welding equipment and also frequently requires replacement.
So-called "touchwork systems" for automatically starting the welding operation when the consumable wire electrode touches the workpiece are known in the art. Such systems usually employ means for maintaining the welding contactor energized during the welding operation and thus avoid the disadvantages of the torch switch. However, in these systems, when the operator stops welding, the welding contactor remains energized and the operator may be exposed to the hazards of high voltages from the power supply.
Another problem that is frequently encountered in MIG arc welding using a touchwork system is that when the operator touches the consumable wire electrode to the workpiece, the wire stick out may blow off due to the high surge current that develops when the arc is first struck. This "blow-off" can be an aggravating problem since the operator must go back to the wire feeder and manually feed wire through the torch until enough stick out is present to make a fresh start.